Electrical measurement of urban telephone network transmission lines

Classification of measurements

Electric measurements of UTN transmission lines are executed to determine electric characteristics compliance with the standards, and also to determine character and point of fault on a line. The electric measurements of UTN lines are executed both at direct and alternating currents. Alternating current measurements are executed after direct current measurements and only in case direct current measurements meet standards. Otherwise measurements at alternating current should be executed after elimination of trouble on the line.

The following measurements at direct and alternating currents are distinguished:

- periodic tests, which are executed during exploitation according to a set plan;

- control instrumentations, which are executed after repair-and-renewal operations;

- measurements to test quality of cables and line equipment, which have entered from a builder, before mounting them on a line;

- measurements to locate point of fault;

- acceptance tests,||implement| which are executed at accepting built, reconstructed, fully-repaired lines to exploitation|maintainance|.

Structure and volume|CU| of periodic tests, control instrumentations and acceptance tests is| defined|definite| in proper manuals. All measurements must be executed by devices which had passed|reeve| the proper state|domainal| or department verification.

Direct current measurements

Direct current measurements enable to make a conclusion concerning accordance of the most unstable line characteristics with the set norms. They are:

- electric insulation resistance;

- electric loop resistance;

- ohmic disbalance|unsymmetry|;

- electric capacitance of circuit.

Besides, direct current measurements are widely used for determination of the most widespread damage – an insulation fault.

For a direct current measuring of circuits the purpose-designed portable cable devices have acquired wide use, for instance ПКП-3, ПКП-4 and ПКП-5.

Direct current|electrical| measurements are worth being executed in the following order: an electric|electrical| loop resistance|holdout||c, an ohmic disbalance|unsymmetry|, an|electrical| electric insulation resistance|holdout||insulant|, an electric|electrical| capacitance|C.| of circuit|chn|.

Ohmic disbalance

An ohmic disbalance (a difference of electric resistances of a circuit conductors at direct current Δ R = R_a – R_b) is measured by a dc bridge (Fig. 9.1). The citcuit beginning (the end A) is connected to the terminals 1 and 2, and an earthed shell or a shield are connected to the terminal 3. The opposite end of circuit B becomes short-circuited and earthed. A bridge with fixed ratio of branch resistances equal to 1 is used for measuring. If the bridge is balanced R = R_B = R_a – R_b. The conductor with lower resistance should be connected to the branch, which contains the variable resistor R_B. The bridge won’t be balanced otherwise, in this case the conductors should be swapped using device switch «Line switching».

 

 

 

 

Loop resistance

A loop resistance (an electric resistance of conductors of a two-wire circuit, R_lr = R_a + R_b) is usually measured, using the dc bridge with a fixed ratio of branch resistances (Fig. 9.2). The resistance of the bridge branches is selected to reduce current in the bridge diagonal in which the indicator is connected to zero during measurement. Having balanced the bridge, we can find the value of the loop resistance over the entire length of the circuit.